Radiation sensitive photoconductive member



Aug. 25, 1959 H. DESSAUER ETAL ,3

RADIATION SENSITIVE PHOTOCONDUCTIVE MEMBER Filed March 1'7. 1953 FIG.|

FIG.2

FIG. 3

INVENTOR JOHN H. DESSAUER HAROLD E. CLARK Y By W ATTORNEY United States Patent RADIATION SENSITIVE PHOTOCONDUC MEMBER John H. Dessauer, Pittsford, and Harold E. Clark,

Rochester, N.Y., assignors to Haloid Xerox Inc., a corporation of New York Application March 17, 1953, Serial No. 342,856

21 Claims. (Cl. 96-1) This invention relates in general to new members sensitive to activating radiation and in particular to photosensitive members of a type which are adapted to electric recording of images. i

In xerography as originally disclosed in Carlson US. Patent 2,297,691, there is employed a member sensitive to activating radiation such as light or photon-type radiation and generally comprising a photoconductive insuiating layer disposed on a conductive backing member. According to that invention, an electrostatic latent image is placed on this member by the selective conduction or dissipation of an electrostatic charge by the action of activating radiation such as a light or optical image either of the visible or invisible spectra on the photoconductive layer. Usually this is accomplished by placing a uniform electrostatic charge on the layer and exposing the charged layer to an optical image, whereby the layer becomes selectively conductive in the activated area.

The general problems involved in radiation-sensitive members start with the requirement that the members support an electrostatic charge for a finite time, preferably a long time, and that they support this charge with a relatively small degree of charge dissipation in the absence of activating illumination. The next requirement of the photosensitive member is that such member must become comparatively highly conductive on exposure to illumination so that the electrostatic charge is rapidly dissipated upon such exposure. A further, closely related and yet distinctly diiferent requirement isthat the charge dissipation be largely complete upon full exposure so that the charge on a depleted area will be of relatively low and preferably substantially zero potential. A next requirement is that the various significant properties be substantially retained from one cycle of operation to the next; that is, that the charge retention, photoconductivity and completeness of charge dissipation be functions of the photosensitive member and its exposure, rather than functions of unrelated conditions such as rapidity of process cycling or the number of cycles which have been accomplished. It has now been found in 'accordance' with the present invention that the new combination member described herein operates in a superior fashion to improve the performance of the member in these categories, thus constituting a substantially better photosensitive xerographic member,

It is an object of this invention to provide a new and improved photosensitive member for xerography.

It is another object of the invention to provide for xerography a new combination photosensitive member comprising a plurality of layers or junctions operating to support a positive polarity electrostatic charge and to dissipate such charge upon illumination.

It is a further object of the invention to provide an electrostatic photosensitive member comprising a conductive base support having thereover three distinct layers consisting of an insulating or barrier layer, a p-type photo- ICC conductive insulator, and an n-type semiconductor o1 insulator thereover.

Additional objects of the invention and means by which these objects are achieved Will in part be obvious and will in part become apparent from the following specification and from the drawings in which:

Figure 1 is a diagrammatic view of the photosensitive layer according to one embodiment of this invention;

Figure 2 is an enlarged fragmentary diagrammatic view of the member according to Figure 1, bearing an electrostatic charge on its surface;

Figure 3 is a similar view wherein the member being discharged by activating illumination,

In general, the new xerographic member according to this invention comprises a conductive backing member 11, supporting on one surface thereof a barrier or junction layer 12 and, thereover, a p-type photoconductive in sulating layer 13. On the upper or outer surface of this photoconductive layer is an insulating outer junction layer 14.

The conductive base member 11 generally comprises a conductive layer characterized by the ability to conduct electricity for the charging or sensitization of the composite member and to accommodate the release of electric charge upon exposure of the member, thus a member having a specific resistivity less than about 10 ohm-cm, usually less than about 10 ohm-cm. Desirably, this base member 11 is also of suflicient structural strength to provide mechanical support or strength to the photosensitive member, thus making it mechanically suited to operation in conjunction with xerographic machines and apparatus. Thus, for example, this base member 11 may comprise a metallic plate, Web, foil, or the like. or, if desired, such suitable member as a conductive plastic, conductive glass, conductive paper, or similar member, all of which, desirably may be in the form of a plane or cylindrical surface or another shape. From a theoretical point of view, the backing member is one having electrons or electrical carriers available in conduction energy bands. The backing member as desired may be relatively rigid as in the case of a metallic plate, cylinder, or the like or may be relatively flexible as with a metallic foil, .21 plastic web, or similar member. In general, the electrical conductivity of this support member must be relatively high as compared with the electrical conductivity of the layers coated thereon. This, of course, does not imply a high conductivity as contemplated by electrical equipment in general since the conductivity of the other members and layers is extremely low. Thus,desirably, the backing member should have a specific resistivity lower than about 10 ohms centimeter, preferably lower than about 10 ohms centimeter. As will be apparent to those skilled in the art, the one essential characteristic of this member is that it must be sufficiently conductive to permit the flow of electricity through the member during certain of the steps of xerography. It is further necessary as a practical matter that the member have the necessary structural strength to act as a support member as well as a conductive backing. 0

On an historical basis the next member of significance is the photoconductive layer 13 which corresponds to the photoconductive insulating layer disclosed by Carlson in the earliest patents on xerography. This layer has certainly necessary and desirable properties, chief of which are, of course, that it must act as insulator in the absence of light or activating radiation and must be significantly conductive in its presence. Whereas in the case of theearly Carlson patents the member was usually charged to negative polarity for sensitization, now in the usual case the new xerographic member is specifically intended to be operated with charging of the photoconductive layer combination to a positive polarity, with the result that the photoconductivity of this layer is a far from simple matter when viewed theoretically and when examined experimentally; In the first place, it is now understood for numerous reasons that this photoconductive insulating layer 13 generally is a p-type conductor rather than an n-type conductor, and, of course, in the absence of illumination or activating radiation it should desirably be almost the perfect insulator. It can be shown by theory and demonstrated in operation that other requirements of this layer are that it should be particularly, a good carrier for electron holes or positive charges of electricity and that the free path of such holes within the boundaries of the layer should be relatively long These general characteristics can be produced adequately in vitreous appearing layers of selenium which are believed to be substantially amorphous seleniumtogether with sub-crystalline formations, nucleation points, or the like. Substantial variations in the physical size and thickness of this layer may be made within the scope of this invention, and the invention accordingly is not to be limited to particular thickness specifically described in the examples. Generally however, this layer 13 will have a specific resistivity greater than 10 ohm-cm. in the absence of illumination, generally in the order of at least about 10 ohm-cm., dropping at least several orders of magnitude in the presence of activating illumination. The layer generally will support an electric potential of at least about 100 volts in the absence of radiation, and generally will be in the order of about 10 to about 200 microns thick, preferably between about 20 and about 80 microns thick.

' Disposed between the backing member 11 and this ptype photoconductor '13 isan intermediate barrier or junction layer 12 which serves several functions. Thus, selenium layers and other photoconductors under applied fields in the order of greater than 10 volts per cm. and generally in the order of 10 and 10 volts per cm. are significant carriers of electricity in that they permit migration of holes substantially through the thin layer. The functions of this member in terms of results achieved are, first, to reduce potential leakage in the absence of activating radiation, which leakage is known in the art as dark decay while supporting charge dissipation in the presence of such radiation, and, second, to reduce, inhibit, and frequently eliminate variation in performance upon repetitive cycling which variation is known in the art as fatigue. The theoretical mechanisms by which these results are achieved are not understood with certainty, but it is believed that they conform with the following theory of operation.

To a limited extent the role of this. layer might be stated as the interposition of a dielectric film which, assuming it to have virtually infinite resistivity, prevents the passage of electrons from the backing member to the. photoconductive layer and hence prevents the loss of charge in such layer which loss would take place by n-type conduction through the photoconductive layer which seldom, if ever, is actually completely lacking in such a p-type conductor. This barrier or junction layer generally is only a small fraction of the thickness of the layer 13 and thus the potential across the thickness of the junction layer is always a relatively small one. For example, if layer 13 be in the order of 50 microns in thickness and the junction layer 12 be in the order of one-half micron thick, then the potential through this junction layer will be in the order of of the potential through the entire photoconductive layer combination. Thus, on such a case if a potential in the order of 500 volts is applied to the xerographic plate, the potential across this junction layer will be in the order of 5 volts, assuming approximately equal dielectric constants.

Referring now to Figure 2, in the drawings it is observed that the combination'layer according to this invention, when having surface charge of positive polarity,

4 is characterized by apparent or effective charge distribution within the layers as there illustrated, namely, a positive polarity charge on the surface of the uper layer 14 to be hereinafter described, coupled with positive-negative migration within layer 13, whereby apparently or effectively, negative charge is concentrated at the upper surface of layer 13 and positive charge is concentrated at the lower surface thereof. This positive charge concentration at the lower surface of layer 13 is, of course, accompanied by a negative charge concentration at the upper surface of the conductive backing member 11. It is apparent from the figure, therefore, that although the potential across layer 12 is relatively small, this potential can be regarded as resembling the potential caused by a concentration of negative charges directly on one side of the layer and a concentration of positive charges directly on the opposite side. According to Holm, Journal of Applied Physics, vol. XXII, pp. 569-574, a charge concentration of this sort across an extremely thin space such as layer 12 is extremely sensitive to variations in thickness and acts according to a pattern of behavior known as the tunnel effect. Thus, an individual electron may be regarded as being drawn across layer 12 by the attraction of one or more closely positioned positive'charges, and when it is drawn therethrough it need not be combined with or directly neutralize such positive charge but instead may be carried into the body of layer 13, thus forming a negative carrier within such layer. The formation of this negative carrier it is observed, is not accompanied by equivalent discharge or neutralization of the positive charge concentration along the lower surface of layer 13, with the result that this is a continuing process involving passage of electrons from the backing member 11 into layer 13 with, ultimately, a substantial neutralization of the positive polarity charge on the xerographic member as a whole.

If layer 12 be of the proper thickness a threshold condition sets in wherein this discharging process is not accomplished. The situation then arises that a positive polarity potential from a small migration of charge can be supported on the combination layer without potential dissipation caused by this particular factor. As an empirical or experimental fact it has been found that the new photosensitive members containing this layer as described herein are particularly characterized by the absence of. the defect known as dark decay and it is reasonably understood and believed that the reason for the absence of dark decay is a function of layer 12 operating in accordance with the theoretical explanation established here.

It is fundamentally essential that this layer exist over the entire active area; that is, that it be substantially free from significant bare or open areas. Beyond this, it is important that the area be thick enough to prevent substantial dark decay and fatigue. For this purpose it has been found that an oxide layer such as a layer of alumintun oxide formed on an aluminum surface should be in the range of about 25 to 200 Angstrom units thick. Similarly, an insulating resin layer such as, for example, a polystyrene layer in the order of 0.1 to 2 microns, preferably between about /3 and one micron has been found desirable.

In the case where the memberdescri-bed herein is exposed to activating illuminationor other radiation, the situation just described is modified to a substantial extent. In this case the migration of charges within the layer 13, which is comparatively conductive, is relatively complete with the effective result that a great proportion of the applied charge acts directly across the junction layer 12. This exceeds the barrier effect of the layer and, of course, causes substantially immediate and substantially complete equalization of the charge on the plate until the residual charge is such that the potential across layer 12 again is reduced to be'in the order of its threshold potential. Since this threshold potential in absolute values is comparatively small, and since the i1- luminated plate concentrates much of its effective potential across layer 12, it is apparent that, under illumination the final residual potential on the entire photosensitive member becomes extremely small and for all practical purposes virtually zero.

From a theoretical point of view, the situation, of course, is not uncomplicated. While layer 13 may be of either p-type or n-type conductivities, the case of a selenium layer will be examined and analyzed in greater detail for purposes of illustration. In the particular case of selenium layers, that the body of layer 13 is a p-type photoconductive insulator and that, as such, it contains sub-molecular or sub-atomic structures in which electrical carriers can be created in the form of positive holes or in the form of electron-hole pairs capable of release or activation by appropriate stimuli. The exact nature of such carriers cannot be expressed since it is believed that they arise from various sources. For example, impurity substituents in a molecular or solid state structure may include electron combination either less than or in excess of the normal electron complement of the structure in which they exist, in which case thermal or photon energy may excite an electron or a hole to a conduction energy band. Likewise, electrons or holes thus excited can subsequently fall back into relatively loosely bound or temporary energy levels known as traps from which they may later be released.

When this concept is applied to layer '13 as contemplated herein, it is apparent that conductivity is imparted to the layer by the photon excitation of suitable carriers, and this conductivity leads to discharge of the charging potential supported on the xerographic member. However, as a result of the discharging cycle, there may, be formed in the layer a number of trapped carriers that are almost indetectable in terms of electric potential but are significant in terms of sources of subsequent conductivity. Thus, if the layer containing these trapped carriers is subsequntly charged to a high potential, these trapped carriers may be thermally or otherwise released to form a small but significant number of carriers capable of migrating within the layer. This gives rise to the situation illustrated in Figure 2 in which migration of carriers follows from re-charging and in which the tunnel efiect becomes significant and may lead to the property known as fatigue which is defined as substantially increased dark decay in subsequent or repeated cycles. 1

The situation which follows upon exposure to activating illumination is entirely different. In this case extremely large numbers of carriers are released, the number being related to the number of photons being absorbed by individual molecular or atomic structures. As shown in Figure 3, the photon-activated carriers migrate in the direction of the arrows under the influence of the applied field and produce at the boundaries of the layer 12 a substantially increased efiect with appropriate migration of carriers through the layer. c The noteworthy result is that layer 12 acts apparently in two entirely difierent ways under the two difierent conditions. Thus, in the absence of illumination the layer substantially prevents charge dissipation either with or without fatigue-induced trapped carriers while it has virtually no measurable efiect on photon-induced charge dissipation.

In the new photosensitive member according to this invention, there is an upper or outer barrier or junction layer 14 disposed on the upper surface of photoconductive layer 13. In cooperation with the p-type photoconductor which forms layer 13, this new layer 14 of semiconductor or insulator servm functions as compared with layer 12. which are comparable in theoretrical derivation although entirely different in practical effect. The function of layer 12 is in effect to prevent penetration to the photoconductive layer 13 from the conductive backing member. oppositely, the function of upper layer 14is to accept a charge of electrons or holes while preventing penetration of such charges into and through layer 13 prior to photon activation. It is immediately apparent that depositing positive charges on the surface of layer 13 by corona discharge or other'method of charging the photosensitive :for sensitization might result in the injection of such positive charges into the p-type conductor. Since an analysis of the nature of the photoconductor 13 reveals that it may be a good carrier of positive electron holes when injected therein, it follows that the eifect of layer 14 can be critical in allowing the combination member to accept and retain a charge of positive polarity when such charge is imposed directly on the surface.

Referring again to Figure 2, it is observed that a positive polarity of electrostatic charge deposited on the surface of layer 14- induces a charge migration as illustrated therein with a concentration of positive charges at the upper boundary of layer 14 and a concentration of positive charges at the lower boundary thereof adjacent to photoconductor 13. This, however, is an induced concentration or migration of electrons and holes and is not an injection of such electrons or holes into the photoconductor. The result of this is that there is an available concentration of positive charges adjacent to the upper boundary of layer 13, but there is not an injection of such charges into the photoconductor in such a manner as to promote substantial p-type conductivity in the absence of illumination. In this manner, the combination of layers 12 and 14 protects photoconductor 13 from injection of conductivity causing charges, both from above and from below, and results in a photosensitive member characterized by the ability to support a charge of high field strength with a minimum of charge dissipation in the absence of illumination. By direct examination, of course, it is apparent that illumination or radiation of the photoconductor 13 activates this layer causing it to be conductive and thereby causing migration of appropriate charges through the boundary layers 12 and 14 with resulting dissipation of a deposited positive polarity electrostatic charge. Generally this layer will be thinner than one micron usually in the order of about 50 to 500 millimicrons or the thickness remaining upon polishing a soft film on a surface such as the photoconductive layer.

Analyzing this top or superficial layer from the point of view of its theoretical efiiect or mechanism of action, it is thought that this layer, which is best considered as an insulator for the polarity to which the member is charged, may act to supply traps for electrons or holes, from which traps the carriers can be appropriately released into the photoconductor body upon illumination with activating radiation.

In the case of a layer which reduces dark decay on a positively charged member andincreases (impairs) dark decay on negatively charged members, the former action can be ascribed to the insulating properties of the layer while the latter suggest the presence of virtual energy levels at or above the level of the conduction band of thephotoconductor layer. The converse case suggests a converse case of energy levels, that is, suggests the presence of filled levels at or below the filled selenium band where the negatively charged member is characterized by high dark decay. Thus, for example, a series of members was made with an aluminum oxide barrier layer 12 according to the general procedure of Example 3 and different top layers 14 were placed on different plates of this series. These plateswere tested for their charge retention with both positive and negative polarity charge on the surface. The test procedure comprised charging the plate, allowing it to stand for two minutes to permit change decay to take place. By comparison with like plates havingno layer 14, it was recorded that a polystyrene layer improved the test reading by 50 volts for positive charging and 15 volts for negative charging;

Tarosimmodified phenol formaldehyde resin improved thereading by 70 and 20 volts, respectively; whereas other film s improved the test reading for positive changing and adversely affected it for negative charging as follows: a melamine-formaldehyde resin +40 volts, 10 volts; nigrosine +60 volts, -l20 volts. One material 'tested'showed the opposite effect: a commercial varnish preparation believed to be based on an alkyd resin, -120 volts for positive charging and +80 volts for negative "Qharging. In each of these'cases a plus value is an indibation of less charge dissipation in absence of illumination and a minus value shows greater charge dissipation. Example 1.-A mirror-finished chromium plate was prepared for a coating operation by thorough cleaning, first with water containing a small quantity of detergent, followed by a solvent wash in isopropanol, followed again by a vapor degreasing in isopropanol vapors. A thin layer of polystyrene approximately 1 micron in thickness, was deposited over the surface of the chromium plate by spraying on the surface a solution in a volatile hydrocarbon solvent and evaporating the solvent by air drying. Immediately thereafter, the polystyrene hearing plate was placed in a vacuum evaporator at a pressure of about /2 micron of mercury and selenium evaporated thereon while the plate was maintained at a temper ature of 75 C. plus or minus 1". The evaporation rate was adjusted so that a 20 micron layer of selenium was deposited in about 10 minutes. After completion of the evaporation, the plate was cooledto room temperaturefi, air was admitted to the chamber, and the plate then removed. A carbon pigmented rosin-modified phenol-formaldehyde resin available under the name Amberol F-70 pigmented with charcoal, was mixed with steel beads about 30 mesh size and the mixture was rolled back and forth across the plate having the selenium coating thereon. After this mixture was rolled back and forth about 5 to times there was on the surface of the plate an extremely thin layer believed to be a layer of the rosin-modified phenol-formaldehyde resin. The resulting product was the xerographic plate of the present invention comprising the metallic backing plate, the polystyrene coating directly thereover, the selenium photoconductive body positioned directly on the polystyrene coating, and the overlayer believed to be a thin layer of rosin-modified phenol-formaldehyde resin. 7 To demonstrate its suitability for xeropgraphy the plate was subjected to the following tests: The plate was charged by corona discharge means to impose on the surface a potential charge in the order of 500 volts positive polarity. The charged or sensitized plate was exposed to a test optical image consisting of typed subject matter and the exposed plate was developed by conventional methods. The resulting image was transferred to a paper web to yield a high quality xerographic print.

As a second test to determine fatigue a portion of the plate was simultaneously flooded with light and subjected to electrostatic charging from an adjacent positive polarity corona discharge electrode. This combined charging and exposing was continued for 10 seconds whereupon the light wascut off. After 60 seconds lapse of time, the entire plate surface was charged to positive polarity by corona discharge. The original charge potential deposited on the plate was measured about 1 second after the charge was deposited, the measurement at all positions being made exactly the same period of time after the charging of the area in question. Subsequently, 60 seconds after charging, the charge potential on the areas was again measured and the following inter pretation made of the data. The difference in potential after the 60 seconds interlude of standing in the dark compared with the original potential expressed in percentage figures was recorded as the dark decay of the plate. The simultaneous flooding and exposure to 8 ligh is considered an approximate test condition to de= termine plate fatigue, and the ratio of initial potential in the thus treated area as against initial potential in the remaining area was recorded as the fatigue index of the plate. The dark decay of the plate thus tested was less than 7.0% and the fatigue index was higher than 0.98, these being excellent dark decay and fatigue readings in comparison with test plates not bearing layers 12 and 14. 1

Example 2. -For test comparative purposes the procedure of Example 1 was repeated with the following differences and alterations; in plate 2A the polystyrene layer was omitted. In plate 2B the coated plate was not treated with the rosin-modified phenol-formaldehyde resin composition. With respect to plate 2B the original potential wassubstantially lower, in the order of about 300 volts, and the potential after 60 seconds was less than 50 volts without regard to whether the plate had been preliminarily fatigued. 'It is considered that plate 2B is unable to accept a significant positive polarity electrostatic charge and maintain such charge for a significant period of time. In the case of plate 2A the initial potential was somewhat lower than in the plate according to Example 1 and was in the order of about 450 volts. The potential after 60 seconds was about volts and the potential in the fatigued area after 60 seconds was about 25 volts. It is considered that plate 2A is characterized by a high dark decay and by excessive fatigue.

Under microscopic examination plate 28 and the plate of Example 1 were found to be indistinguishable and it is concluded that the top layer, believed to be a layer of rosin-modified phenol-formaldehyde resin, is extremely thin, and can be detected best by its electrical characteristics and its electrical modification of the plate.

Example 3.A mirror-finished aluminum plate was immersed in a hot aqueous solution of orthophosphoric acid and nitric acid until the plate was free of any detectable aluminum oxide film. The clean plate was thoroughly washed with distilled water and was then bakedfor 1 hour at 250 F. It is understood and believed that as a result of the cleaning and subsequent baking treatment a highly uniform thin layer of amorphous aluminum oxide is formed on the aluminum plate as a layer in the order of'about 100 Angstroms thick.

The thus prepared plate was coated with vitreous appearing selenium by Vacuum evaporation under a pressure of 0.5 micron of mercury while maintaining the plate at a temperature of 75 C. A layer 20 microns thick of vitreous selenium was deposited in 10 minutes, after the plate was cooled and removed from the vacuum chamber. The surface of the plate was then polished with montan wax and the excess was removed by vigorous polishing with a clean cloth, leaving a thin wax film on the surface. The resulting plate was a highly satisfactory and superior xerographic plate and was characterized by a 'dark decay of less than 10% and a fatigue index of greater than 0.98.

Example 4.-A smooth surfaced brass plate was pre pared for coating by thorough cleaning employing mechanical washing with water containing a small amount of a detergent followed by rinsing with clear water. The clean plate was then polished with a parafiin wax. After application of the polish the surface was thoroughly rubbed with a clean dry cloth, leaving the surface highly polished and apparently with an extremely thin layer of a Wax or the like believed to be a hydrocarbon 'wax composition.

'After cleaning and polishing the brass plate was coated with a selenium layer SO microns thick by vacuum evap oration under a pressure of 0.5 micron mercury while maintaining the temperature at about 83 C; The vacuumevaporation was controlled to give a layer of vitreous appearing selenium 50 microns thick in a periodof'time of 10 minutes. After completion of the evaporation, the plate was removed from the vacuum chamber and immediately polished with a silicone wax base polish comprising a silicone wax in an organic solvent. The resultmg xerographic plate was of excellent quality for continuous tone or line images and by the described tests had a dark decay less than and a fatigue factor of at least 0.98. When comparably treated members were made including the additional step of removing the wax coating by vapor degreasing, high dark decay and poor fatigue factors were recorded.

Example 5 .-A stainless steel plate was thoroughly cleaned by washing with soap and water followed by washing with water containing a small quantity of a commercial detergent after which it was thoroughly rinsed with water. Thereafter, a thin film approximately 500 millimicrons thick was cast on the surface from a solution of polymethyl methacrylate in ethylenedichloride solution. This film was thoroughly air dried, whereupon it was coated with a layer of 40 microns of vitreous appearing selenium according to the method of the previous example. A solution of paraffin in iso-octane was spread on the surface and polished off, finishing with a clean, dry cloth which is believed to have left on the surface an extremely thin layer of paraifin and which provided the plate with a smooth, even, shiny finish. The plate, according to the previously described tests, was a highly superior xerographic plate.

In the preparation of the xerographic plates according to this invention, it is extremely critical that the first layer or layer 12 as shown in Figure 2 must be a thin layer and must critically be extremely smooth and regular throughout its entire area. In general, it is important that this layer be a good dielectric and extremely good insulator so that it is capable of strictly limiting the migration or passage of electrons therethrough. The layer, however, should be relatively thin since it is desired that this layer shall not contribute substantially to the residual charge potential remaining on the plate after charging and complete exposure to light. In this connection it is observed, however, that the layer has a somewhat anomalous behavior when it is directly coated with a photoconductive layer 13. Thus, in the case of the plate described in Example 1, the residual potential of the composite plate was substantially zero in spite of the fact that the metallic base, having the polystyrene coating thereon without the photoconductive coating placed thereover, was capable of accepting and retaining a positive charge potential greater than 50 volts either with or without exposure to illumination. This same base, however, when coated with the photoconductive layer, had a residual potential greatly reduced below that which could be supported by the polystyrene layer acting by itself and in the absence of. the photoconductive layer.

It is apparent that there is relatively wide freedom of choice in the selection of the insulating layer 12 to be placed directly on the conductive backing. Thus, insulating resins of various types may be employed including polystyrene, butadiene polymers, and copolymers prepared from butadiene, acrylic and methacrylic polymers and co polymers prepared from acrylic and methacrylic bases as well as ureaformaldehyde and melamineformaldehyde resins and modified resins, vinyl resins, alkyd resins, and cellulose base resin and plastic compositions, hydrocarbon and like waxes, oils, etc., and other organic compounds characterized by good insulating properties. In addition, theremay be employed inorganic layers, such as, for example, oxides, sulfides, and the like, which may be coated ,on a suitable base, and in a particularly desirable embodiment of the invention these may be oxides or sulfides of the metallic base directly coated on such a base. If desired, special types of xerographic plates may be employed in t is manner, such as, for example, by employing compounds of heavy metals and the like, such as lead resinates,

10' lead sulfides, etc., as this layer,whereupon intensification of X-rays and penetrating radiation may be achieved.

It is to be realized, of course, that the theory herein set forth is in the form of theory rather than proven fact, but as a necessary corollary it follows that existing proven facts support the theory. It is to be realized, therefore, that the theory is presented according to present belief and understanding but that it is not intended to limit the scope and nature of the. invention to the complete correctness of the theory posited herein. Thus, specifically, photosensitive members accordingto this invention. have been prepared with pains being taken to exclude in one case and include in the other the insulating or dielectric layer designated 12 and to exclude in one case and include in the other the top or junction layer designated 14. The behavior of such plates with and without activating illumination and under positive and negative surface charging of the photosensitive member is in accord with the theory set forth herein and has established factually that these two layers contribute significantly to improved performance of photosensitive layers for xerography.

As shown in the examples, the presently preferred photoconductor layer is seleniumin its vitreous appearing form, which is believed and understood to consist of amorphous selenium probably with subcrystalline nucleation points or the like. Such a layer may be prepared by various methods including vacuum evaporation, casting and whirling to achieve a smooth surface, spraying and polishing, pressing and the like, and including coating with a pigmented lacquered base wherein the pigment is amorphous or vitreous selenium. It is to be understood, of course, that other photoconductors may be employed, including such photoconductors as sulfur, anthnacene, mixtures of selenium and sulfur, and the like, and also including other photoactive materials characterized by the activation of electrons with a light. Included in this class are the generally known phosphor and phosphor-like materials such as cadmium sulfide, zinc sulfide, chemically activated photoconductors, and other organic and inorganic compounds including mixtures and various combinations thereof.

The upper layer or layer 14 of the new composite xerographic plate appears desirably to be an extremely thin layer and as understood at present should be an insulating material or one acting as an insulator under the polarity to which the new member is charged. Various waxes, hydrocarbons, and resins can be employed for this layer and have been employed in the form of layers of indetectable thickness except when measured by electrical characteristics imparted to the xerographic plate. Suitable materials are hydrocarbon and animal and vegetable waxes, insulating resins such as polystyrene, urea-, phenol-, and melamine-formaldehyde resins, vinyl resins and the like. In addition to the employment of films placed on the surface of the photoconductor it is to be understood that the upper boundary of the photoconductive layer itself may be converted to such a film. Thus, it is believed and understood that the outer surface of a. selenium zerographic plate having the desired layers 12. and 13 according to Figure 2 may be treated with an oxidizing agent apparently to yield a top layer 14 of selenium dioxide which appears to behave in accordance with the be.- havior pattern described herein.

The present invention has been described with reference to certain specific embodiments which have been presented in illustration of the scope of the invention. It is to be understood, however, that numerous variations of the invention may be made and such variations are well within the intended scope and spirit of the invention.

These variations may include physical variations in shape of the xerographic photosensitive member as well as variations in the selection of the ingredients employed in the preparation of such new members. Specific types or forms of photosensitive members may be developed for specific uses and purposes, and itisto be understoodtha} 7 said barrierflayer, and a separately formed layer about 0.05 to about 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material less than about 2 micron's'thick adapted to substantially prevent charge dissipation' in the absence of activating the photoconductive insulating layer and adapted to permit substantially complete charge dissipation under the influence of activating the photoconductive insulating layer, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

2. A Xerographic photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base layer, a non-particulate layer of vitreous photoconductive insulating selenium about -200 microns thick perma- 'riently afiixed on said barrier layer and a thin separately formed layer about 0.05 to about 1 micron thick disposed on the outer surface of said layer of vitreous photoconductive insulating selenium, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating the photoconductive insulating selenium and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

'3. A xerographic photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base layer, a non-particulate layer of vitreous photoconductive insulating selenium between about 10 to about 200 mi- 'crons thick' permanently affixed on said barrier layer and a separately formed electrically insulating uniform layer about 0.05 to about 1 micron thick disposed on the outer surface of said layer of vitreous selenium, the barrier layer being a body of electrically insulating material between about0.l and about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating said layer of vitreous selenium and adapted to permit substantially complete charge dissipation under the influence of activating said layer of vitreous selenium, and the outer layer being a body of electrically insulating material incapable of supplying charge carriers by absorption of visible light.

4. A photosensitive member comprising an electrically conductive cylindrical base layer, a separately formed barrier layer directly overlying the outer surface of said base layer, a non-particulate layer of vitreous photoconductive insulating selenium between about 10 and 200 microns thick permanently affixed on said barrier layer and a separately formed uniform layer about 0.05 to about 1 micron thick disposed on the outer surface of said layer of vitreous selenium, the barrier layer being a body of electrically'insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating said layer of vitreous selenium and adapted to permit substantially complete charge dissipation under the influence of activating said layer of vitreous-selenium, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

5. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a'surface of said base layer, a nonparticulate layer of photoconductive insulating material between about 10 and 200 microns thick permanently afiixed on said barrier layer and a separately formed outer layer about 0.05 to about 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a layer of electrically insulating resin between about 0.1 and about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating said photoconductive insulating layer and adapted to permit substantially complete charge dissipation under the influence of activating said photoconductive insulating layer, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

6. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of the base layer, a nonparticulate photoconductive insulating layer permanently aiiixed on said barrier layer, and a separately formed layer about 0.05 to about 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material adapted to substantially prevent charge dissipation in the absence of activating the photoconductive insulating layer and adapted to permit substantially complete charge dissipation under the influence of activating the photoconductive insulating layer, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

7. A photosensitive member comprising an electrically conductive cylindrical base layer, a separately formed barrier layer directly overlying the outer cylindrical surface of said base layer, a non-particulate photoconductive insulating layer between about 10 and 200 microns thick permanently afiixed on said barrier layer, and a separately formed layer about 0.05 to about 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating the photoconductive insulating layer and adapted to permit substantially complete charge dissipation under the influence of activating the photoconductive insulating layer, and the outer layer being an electrically insulating layer incapable of supplying charge carriers by absorption of visible light.

8. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base layer, a nonparticulate layer between about 10 and 200 microns thick of a photoconductive insulating material having a long range for positive charge carriers said layer of photoconductive insulating material being permanently afiixed on said barrier layer and a separately formed thin semiconductor layer disposed on the outer surface of said photoconductive insulating body, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being a body of n-type semi-conductive material capable of supplying charge carriers by absorption of visible light.

9. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base, a non-particulate layer of photoconductive insulating material between about 10 and 200 microns thick having a long range for electrons permanently afiixed on said barrier layer, and a thin separately formed p-type semiconductor layer dis posed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation and the outer layer being a body of semi-conductive material capable of supplying charge carriers by absorption of visible light.

10. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base layer, a nonparticulate layer of photoconductive insulating material having a long range for positive charge carriers and being permanently atfixed on said barrier layer, and a thin separately formed n-type semiconductor layer disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being a body of n-type semiconductive material capable of supplying charge carriers by absorption of visible light.

11. A photoconductive insulating member comprising an electrically conductive base layer, a separately formed barrier layer directly overlying a surface of said base layer, a non-particulate layer of photoconductive insulating material having a longrauge for electrons and being permanently aflixed on said barrier layer and a thin separately formed p-type semiconductor layer disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being a body of p-typesemiconductive material capable of supplying charge carriers by absorption of visible light.

12. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer overlying a surface of said base layer, a non particulate layer between about and 200 microns thick of a photoconductive insulating material having a long range for positive charge carriers, said layer being permanently affixed on said barrier layer, and a separately formed outer layer betweenabout 0.05 and 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of ac tivating radiation, and the outer layer being a layer of ntype semiconductive material capable of supplying change carriers by absorption of visible light.

'13. A photosensitive member comprising an electrically conductive base layer, a separately formed barrier layer overlying a surface of said base layer, a non-particulate layer between about 10 and 200 microns thick of a photoconductive insulating material having a long range for electrons said layer being permanently afiixed on said barrier layer, and a separately formed outer layer between about 0.05 and 1 micron thick disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being a layer of p-type semiconductive material capable of supplying charge carriers by absorption of visible light.

14. A photosensitive member adapted for electrophotography wherein the member is charged to positive polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an electrically conductive base layer, an electrically insulating separately formed resin barrier layer between about 0.1 and 2 microns thick directly overlying a surface of said base layer, a nonparticulate layer of photoconductive insulating material having a long range for positive charge carriers, said layer of photoconductive insulating material being permanently affixed on said barrier layer and a separately formed outer layer of an n-type semiconductor material capable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05 and about 1 micron thick and disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation.

15. A photosensitive member adapted for electrophotography wherein the member is charged to positive polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an electrically conductive base layer, an electrically insulating separately formed resin barrier layer between about 0.1 and 2 microns thick directly overlying a surface of said base layer, a nonparticulate layer of vitreous photoconductive insulating selenium between about 10 and about 200 microns thick permanently afiixed on said barrier layer and a separately formed outer layer of an n-type semi-conductor material capable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05 and about 1 micron thick and disposed on the outer surface of said layer of vitreous selenium, the barrier layer being a body of electrically insulating material adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation.

16. A photosensitive member adapted for xerography wherein the member is charged to positive polarity comprising an electrically conductive cylindrical base layer, a separately formed barrier layer directly overlying the outer curved surface of said cylindrical base layer, a nonparticulate layer of vitreous selenium about 10 to 200 microns thick permanently afiixed on said barrier layer and a separately formed outer layer about 0.05 to about 1 micron thick disposed on the outer surface of said selenium layer, the barrier layer being a body of electrically insulating material less than about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer being a body of n-type semiconductive material capable of supplying charge carriers by absorption of visible light. 17. A photosensitive member adapted for electrophotography wherein the member is charged to positive polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an aluminum base layer, a separately formed aluminum oxide barrier layer directly overlying a surface of said base layer, a non-particulate layer of photoconductive insulating material having a long range for positive charge carriers, said. layer of photoconductive insulating material being permanently aifixed on said barrier layer, and a separately formed outer layer of an n-type semi-conductor material capable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05 and about 1 micron thick and disposed on the outer surface of said photoconductive insulating body, the barrier layer being a layer between about 25 and about 200 Angstrom units thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation.

18. A photosensitive member adapted for electro- '15 photography wherein themember is charged to negative polarity and an electrostatic image is formed thereon by Selective charge dissipation by exposure to a pattern of light and shadow, comprising an electrically conductive base layer, an electrically insulating separately formed resin barrier layer directly overlying a surface of said base layer, a non-particulate layer of photoconductive insulating material having a long range for electrons, said layers being permanently aflixed on said barrier layer and a separately formed outer layer of a p-type semi-conductor material capable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05 and about 1 micron thick and disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electricallyinsulating material between about 0.1 and about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation.

19. A photosensitive member adapted for electrophotography wherein the member is charged to negative polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an electrically conductive base layer, an electrically insulating separately formed resin barrier layer directly overlying a surface of said base layer, a non-particulate layer of photoconductive insulating material having a long range for electrons, said layer of photoconductive insulating material being between about 10 and about 200 microns thick and being perma nently afiixed on said barrier layer. and 'a separately formed outer layer of ap-type semi-conductor material capable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05 and about 1 micron thick and disposed on the outer surface of said photoconductive insulating layer, the barrier layer being a body of electrically insulating material between about 0.1 and about 2 microns thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted to permit substantially complete charge dissipation under the influence of activating radiation.

20. A photosensitive member adapted for electrophotography wherein the member is charged to positive polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an aluminum conductive base layer, a separately formed aluminum oxide barrier layer directly overlying a surface of said base layer, a nonparticulate layer of photo-conductive insulating material permanently affixed on said barrier layer and a separately formed outer layer of an electrically insulating material incapable of supplying charge carriers by absorption of visible light, said outer layer being between about 0.05

and about 1 micron thick and disposed on the outer S111: face of said photoconductive insulating layer, the barrier layer being a body of electrically insulating materialabetween about 25 and about 200 Angstrom units thick adapted to substantially prevent charge dissipation in the absence of activating said photoconductive insulating layer and adapted to permit substantially complete charge dissipation under the influence of activating'said photoconductive insulating layer.

21. A photosensitive member adapted for electrophotography wherein the member is charged to positive polarity and an electrostatic image is formed thereon by selective charge dissipation by exposure to a pattern of light and shadow, comprising an aluminum conductive base layer, a separately formed aluminum oxide barrier layer directly overlying a surface of said base layer, a non-particulatevitreous selenium photoconductive insulating layer between about 10 and about '200'microns thick permanently aflixed on said barrier layer and a separately formed n-type. semiconductor layer between about 0.05 and about 1 micronthick disposed on the outer surface of said vitreous selenium layer, the barrier layer being a body of electrically insulating material between about 25 and about 200 Angstrom units thick adapted to substantially prevent charge dissipation in the absence of activating radiation and adapted' to permit substantially complete charge dissipation under the influence of activating radiation, and the outer layer'being a body of n-type semi-conductive material capable of supplying charge'carriers by absorption of visible light.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,731 De Boer et al. Dec. 13, 1938 2,189,576 Brunke Feb. 6, 1940 2,199,104 Johnson et al. Apr. 30, 1940 2,277,013 Carlson Mar. 17, 1942 2,297,691 Carlson Oct. 6, 1942 2,337,329 Hewlett Dec. 21, 1943 2,554,225 Taylor May 22, 1951 2,613,301 Dubar et al. Oct. 7, 1952 2,619,418 Mayo Nov. 25, 1952 2,654,853 Weimer Oct. 6, 1953. 2,659,670 Copley Nov. 17, 1953 2,662,832 Middleton et al. Dec. 15, 1953 OTHER REFERENCES Some Observations on the Photoeflfect in Cadmium Sulphide, Rose et al.; Physical Review; 1949; vol. 76, page 179. (Photostat copy in Div. 67.)

An X-Ray Study of the Structure of Rectifying Sele-t nium Films, Clark et al.; The Electrochemical Society; 1941; vol. 79; pages 355-365; page 359 particularly relied upon. (Copy in Div. 48.)

Disclaimer f2,901,348.-J07m H. Dessauev", Pittsford, and Harold E. Clark, Rochester, N.Y. RADIATION SENSITIVE PHOTOCONDUCTIVE MEMBER.

Patent dated Aug. 25, 1959. Disclaimer filed Oct. 3, 1974, by the assignee, Xerox Corpomzfion.

Hereby enters this disclaimer to claims 121, inclusive, of said patent.

[Ofiicial Gazette Apm'l 2.9, 1975.] 

1. A PHOTOCONDUCTIVE MEMBER COMPRISING AN ELECTRICALLY CONDUCTIVE BASE LAYER, A SEPARATELY FORMED BARRIER LAYER DIRECTLY OVERLYING A SURFACE OF SAID BASE LAYER, A NON-PARTICULATE PHOTOCONDUCTIVE INSULTING LAYER BETWEEN ABOUT 10 AND 200 MICRONS THICK PERMANENTLY AFFIXED ON SAID BARRIER, LAYER, AND A SEPARATELY FORMED LAYER ABOUT 0.05 TO ABOUT 1 MICRON THICK DISPOSED ON THE OUTER SURFACE OF SAID PHOTOCONDUCTIVE INSULATING LAYER, THE BARRIER LAYER BEING A BODY OF ELECTRICALLY INSULATING MATERIAL LESS THAN ABOUT 2 MICRONS THICK ADAPTED TO SUBSTANTIALLY PREVENT CHARGE DISSIPATION IN THE ABSENCE OF ACTIVATING THE PHOTOCONDUCTIVE INSULATING LAYER AND ADAPTED TO PERMIT SUBSTANTIALLY COMPLETE CHARGE DISSIPATION UNDER THE INFLUENCE OF ACTIVATING THE PHOTOCONDUCTIVE INSULATING LAYER AND THE OUTER LAYER BEING AN ELECTRICALLY INSULATING LAYER INCAPABLE OF SUPPLYING CHARGE CARRIERS BY ABSORPTION OF VISIBLE LIGHT. 